PSI - Issue 13

W. Teraud / Procedia Structural Integrity 13 (2018) 238–242

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Teraud W. / StructuralIntegrity Procedia 00 (2018) 000–000

Existing instruments to measure deformations at elevated temperatures can be divided into two groups: those to measure deformations at discrete points of an object and those to measure deformations of a whole object being examined (or to continuously measure deformations at a selected area of a subject). There are high-temperature resistance strain gauges, model НМТ-450 [1]. However, their operating temperature is limited to 450°C, and their size to ~1 cm, that, as compared to the size of a specimen typically not exceeding 5-10 cm, does not make it possible to use them for reliable research of deformation localization. With a background of drawbacks and disadvantages of contact measurement systems, the use of non-contacting methods is required. The TRViewX [2] video-extensometer is designed for non-contact measurements of longitudinal and lateral deformations of a specimen at elevated temperatures. However, this video-extensometer does not enable to measure shift of arbitrarily selected points at a specimen and does not make it possible to completely control form changes of specimens subject to deformation. The use of a method to include a digital camera and a specimen with marks at distances of 10, 20, 30, 40 mm being check points for measurements is described in [3]. In the paper [3] it is not specified whether the method can be used for high-temperature deformation measurements. A method is known [4], where a photo layer is applied onto the surface of a specimen, upon being pre-exposed through a coordinate grid pattern. But the use of this method at elevated temperatures is strongly limited and it is not also possible to measure specimen form changes in the course of deformation. In the paper [5, 6] a method is described to include formation of light line on the surface of a subject (using a light emitting system) lying at a specified section of a subject, light line image acquisition, its processing and positional measurements of a cross-section profile by means of computer-aided image processing. These methods require rotation, they don’t enable to determine surface deformation and require additional visual access for a laser. The shadow method [7] can be used for non-contacting measurements of subject form changes, where boundaries of a shadow area are determined. But this method is not meant for measurements of surface deformations and can’t be used for larger subjects. A distinguishing feature of methods, where structured light is used, lies in special emission projected upon the surface of an object of a specified structure with sinusoidal intensity distribution [8], specified structure of light stream [9] or spatially-modulated by intensity [10]. But the use of these methods at higher temperatures results in emergence of significant difficulties due to natural background to include IR or visible emission of heaters and lighting fixtures. In the paper [11] the known industrial system Vic-3D is used, based upon the Digital Image Correlation method (DIC) [12, 13]. A stochastic pattern must be applied onto a specimen, using a special paint, but this is impossible at higher temperatures. The author of the paper has tested this measurement system (with no paint applied) at elevated temperatures, but the system did not enable to obtain any results required. The purpose of this paper is development of a non-contacting method to measure parameters of geometry and deformation of a heated specimen subject to high-temperature deformation. 2. Method description These are non-contacting measurements of geometry parameters that underlie this method. The specimen geometry was reconstructed using pictures taken during the experiment. Figure 1a shows the basic components of the system. To take pictures of the specimen, one bores a hole through the insulation layer of the furnace and covers that with a piece of Optical Silica Glass. On the outside surface that is not exposed to high temperatures, a camera is installed. The Camera transmits pictures taken to a PC, where they are processed in a specially developed software environment. For further analysis of the surface deformation, reference lines are mechanically applied onto the grinded surface of a specimen – you can see their image on a non-deformed specimen in Figure 1b. To maintain an optimal number of frames within one test, a Controller is created to operate and control a “frames-per-minute” parameter. Illumination inside the furnace thanks to hot heating elements was not sufficient for taking high-quality pictures. The major problem was non-possibility to make a second window through which it could be possible to illuminate the sample. Therefore, three illumination options were developed and tested.